Treatment of metals or alloys



Dec.15,19,7O OURNE ETAL 3,541,712

TREATMENT OF METALS 0R ALLOYS Filed Sept. 23, 1965 United States Patent 3,547,712 TREATMENT OF METALS 0R ALLOYS Alan A. Bourne, Jack C. Chaston, and Alan S. Darling, London, England, assignors to Johnson, Matthey & Company Limited, London, England, a British company Filed Sept. 23, 1966, Ser. No. 581,443 Int. Cl. C22f 1/08, 1/14 US. Cl. 148-11.5 14 Claims ABSTRACT OF THE DISCLOSURE This invention relates to improvements in and relating to the treatment of metals or alloys or metallic compositions for the purpose of imparting improved mechanical properties thereto, particularly increased strength at high temperatures. The invention is more particularly, but not exclusively, concerned with the strengthening of metals of the platinum group, especially platinum and alloys of platinum, and in the following, for the sake of simplicity, reference is directed particularly to the treatment of platinum and alloys thereof, it being clearly understood, however, that no limitation is intended thereby.

Whilst platinum and alloys of platinum have long been recognised as particularly suitable metals for use where resistance to stress under oxidising conditions at elevated temperatures is required, the mechanical properties of these materials per se did not always meet the ever increasing strength requirements of metals, such as is called for by modern technological advances.

In an endeavour to satisfy this demand for increased mechanical properties, various proposals have been made to strengthen the properties of platinum and platinum alloys by the addition thereto, in the form of a dispersed phase therein, of oxides, such as thoria, zirconia, hafnia, titania, alumina or rare earth metal oxides. Improved results were also found to be obtainable by the use of compounds other than oxides, and the present applicants have themselves suggested the use of tungsten carbide as a dispersion-hardening addition.

However, whilst platinum, dispersion-hardened or grain-stabilised in one or other of the above manners, was considerably stronger than pure platinum at elevated temperatures, it was by no means entirely satisfactory, mainly owing to it being brittle. Moreover, the creep life of such material was found to be only some 7 to hours when subjected to a tensile stress of 700 lbs/sq. in. at a temperature of 1400 C. It was, moreover, found that, under conditions of severe stress over long periods at elevated temperature, the strengthened material was liable to fail due to fracture at the grain boundaries, particularly those boundaries disposed at right angles to the direction of applied stress.

With the aim of overcoming the above disadvantages, and enabling strengthened material which is not subject to the aforesaid faults to be obtained, the applicants investigated the problem and have surprisingly found that it can be solved in a simple and highly satisfactory manner.

The principal object of this invention, therefore, is to provide a method of improving the mechanical properties 3,547,712 Patented Dec. 15, 1970 of a metal or metallic alloy or composition which has been strengthened by the inclusion therein of a non-metallic phase, such as, for example, a dispersed oxide or carbide, boride, nitride or silicide phase.

Another object of the invention is to provide a strengthened platinum or platinum alloy or composition having greatly improved mechanical properties.

According to one feature of this invention, there is provided a method of improving the mechanical properties of a strengthened metal or metallic alloy or composition as hereinafter defined, which comprises the steps of subjecting the metal or alloy or composition to coldworking and subsequently annealing at a suitable temperature, the extent of cold-working being such that the recrystallisation effected during annealing results in an elongated crystal or grain structure highly orientated in the direction of working.

In carrying out the invention, the method may be applied to the treatment of a strengthened noble metal or noble metal alloy or a base metal or base metal alloy or of a strengthened composition formed of compacted and sintered noble metal and/ or noble metal alloy powder particles or of compacted and sintered base metal and/ or base metal alloy powder particles or of a mixture of any two or more of the above.

By the expression strengthened metal or metallic alloy or composition, as used herein and in the claims, is meant a metal, alloy or sintered powder composition to which has been imparted strengthened properties by the inclusion therein of a non-metallic phase, such, for example, as a dispersed phase of a refractory metal oxide, carbide, boride, nitride or silicide.

According to another feature of the invention, there is provided a method of improving the mechanical properties of platinum or a platinum alloy or composition which comprises the steps of subjecting a strengthened platinum or a platinum alloy or. composition as herein-before defined to cold-working and subsequently annealing the extent of cold-working being such that the recrystallisation efiected during annealing results in an elongated crystal or grain structure orientated to a substantial extent in the direction of working.

Preferably the cold-working should produce reduction in cross-sectional area of at least The temperature at which the subsequent anneal is carried out will vary with the metal under treatment and can be readily ascertained by experiment. In the case of platinum and platinum alloys, a temperature of about 1400 C. will be found suitable.

The following examples illustrate the manner in which the invention may be carried out in practice, as applied to the treatment of strengthened platinum and rhodiumplatinum compositions, it being understood that the invention is in no way limited to, or by, these examples.

EXAMPLE ii Percent Life at 1,400 0., hours reduction Diameter, inch in area 700 psi. 1,400 p.s.i.

Micro-examination of the recrystallised grain structure showed that the length/width ratio of the grains of the Ms" diameter rod was approximately 5, whilst that of the grains in the 0.040 diameter wire was approximately 12.5.

EXAMPLE II 4 EXAMPLE v Sieved rhodium and platinum powders were mixed in the ratio of one to nine and then milled together for 24 hours with 0.04% of titanium carbide.

This mixture was vacuum-annealed, compacted and vacuum-sintered for 3 hours at 1400 C. After hot forging at about 1100 C. it was then drawn to wire 0.040 diameter, being subjected during the cold-working to a total reduction in area of approximately 97%. Subsequent an nealing at 1400 C. developed a highly elongated grain structure. Creep tests on this material, during which a tensile stress of 700 lbs/sq. in. was applied at 1400" 0., showed a life of approximately 2000 hours.

Tests under similar conditions on rhodium-platinum al- Cold work, total reduction Ingot No. Method of fabrication percent of area Life in hours at 700 p.s.i. and 1,400" O.

4 Ingot cold swaged Micro-examination of the sheet after creep testing disclosed a highly elongated grain structure in material from ingots 3 and 4, significant elongation in sheet from ingot 2, and little, if any, elongation in sheet from ingot No. 1.

EXAMPLE III To a batch of platinum powder, which had been sieved to pass through a 60 mesh B.S, sieve, and washed to remove any soluble trace impurities, 0.04% by Weight of titanium carbide powder, finer in diameter than 5 microns, was added. This powder mixture was then ballmilled dry, in a rubber mill using steel balls, for a period of 24 hours.

This powder mixture, which was hard and difiicult to compact, was then annealed in vacuum for two hours at 800 C. until it became capable of being pressed to bars under a pressure of about 8 tons per square inch. The bars so formed were then sintered in vacuum at 1400 C. for a period of 3 hours, hot forged, and finally cold drawn to wire. The reduction in area accomplished by the cold drawing was approximately 97%. The wire was then annealed at a temperature of 1400 C.

Creep tests carried out on wire produced in the above described manner provided results as tabulated below:

To 15 ounces of sieved and washed platinum powder was added 0.04% of micron grade titanium carbide powder and the mixture ball-milled for 24 hours. This mixture was vacuum-annealed for 2 hours at 800 C. and carefully poured into a steel die As" wide and compacted under a pressure of tons per square inch. The pressing was then sintered at 1400 C. in vacuum for 3 hours, allowed to cool, repressed to 80% of its theoretical density and resintered for 2 hours at 1400 C.

After hot forging in the plane of pressing the bar was then cold rolled to sheet, samples being taken at various stages of reduction. The samples were then annealed at a temperature of 1400 C. to effect recrystallisation.

Creep tests on sheet made from this batch of platinum showed the following results:

CREEP TEST DATA SHEET FROM OUNGE INGOT (1,400" 0., 700 P.S.I.)

loy, strengthened with thoria and treated in accordance with the invention, showed a creep life of only 1000 hours before failure.

In order further to demonstrate the very considerable advantages obtained with the improved material in accordance with the invention, creep tests were carried out on specimens of platinum metal strengthened in various ways and treated in accordance with the method of the invention, together, for comparison purposes, with similar tests under similar conditions on specimens of platinum metal prepared by normal casting and powder metallurgical procedures and also of platinum metal strengtherred by oxide and carbide additions but not treated in accordance with the invention.

In carrying out these tests, the following test specimens were first prepared as follows:

A quantity of platinum powder was sieved through a 60 mesh gauze and then divided into twelve batches A, A F, F of grams, which were separately treated in the following manners:

Batches A, A: Melted and cast platinum ingots Two batches were melted in air in alumina crucibles, cast into ingots of approximately /2 square and one ingot was carefully reduced by cold forging to thick sheet from which the test specimen A was machined. The forging was carried out gently with frequent intermediate annealing. The other ingot was reduced by cold forging, swaging and finally by drawing to 1 mm. wire, Specimen A, a total cold reduction of about 99.5% in area, followed by recrystallisation annealing, in accordance with this invention.

Batches B and B: Consolidated platinum powder Two batches were pressed in a steel die at a pressure of approximately 10 tons/ sq. in. to form rectangular compacts which were vacuum sintered at 1400 C. for two hours. The compacts were then hot forged in air of 1150-1200 C. for a total reduction in area of 40-45% until the theoretical density of platinum had been obtained. One ingot was formed into sheet and a test specimen B was machined from the product. The other forged ingot was cold swaged and drawn to 1 mm. wire and recrystallisation annealed in accordance with the invention, to form Specimen B.

Batches C and C: Platinum sponge with thoria addition To these two batches of platinum powder were added 1.14 grams of thorium nitrate dissolved in 20 ccs. of distilled water. The suspension was mixed to form a smooth paste, dried at 100 C. and then heated for three hours at 800 C. in hydrogen to convert the thorium nitrate to thorium oxide and giving a final content of thoria of about 0.50% by weight. The two powder batches containing finely dispersed thoria were then compacted at a pressure of 10 tons/sq. in. and the compacts sintered for 1 hour at 1400 C. The sintered ingots were then each treated similarly to Batches B and B respectively to provide test specimens C and C' respectively.

Batches D and D: Platinum sponge with titanium carbide addition Each of these batches was made by adding to 100 grams of platinum powder, 0.04% by weight of micron grade titanium carbide powder and thoroughly mixing by tumbling. The mixtures were then compacted into rectangular ingots at a pressure of 10 tons/sq. in. and sintered at a temperature of 1400 C. for two hours. The sintered bars were then hot forged to consolidate them and one was reduced to sheet and the other treated in accordance with the invention to provide Specimens D and D respectively.

Batches E and E: Platinum powder with thoria addition These batches were prepared by adding, to each of two 100-gram batches of platinum powder, 1.14 grams of thorium nitrate dissolved in 20 ccs. of distilled water. The suspension was mixed to form a smooth paste and dried at a temperature of 100 C. The paste was then heated for three hours at a temperature of 800 C. in hydrogen to convert the thorium nitrate to thorium oxide, the final content of which was approximately 0.50% by weight. The two batches were then ball-milled dry in a polypropylene mill with tungsten carbide balls for 12 hours. Pressed compacts produced from this thoria-containing powder mixture were then sintered in hydrogen for 1 hour at 1400 C., repressed to 85% of the theoretical density, resintered for 1 hour at 1400 C. in hydrogen and finally respectively treated as above described to form, respectively, sheet (Specimen E), and cold-worked and annealed wire (Specimen E).

Batches F and F: Platinum powder with titanium carbide addition To each of two 100 grams of platinum powder batches was added 0.04% by weight of micron grade titanium carbide powder, with thorough mixing by tumbling, and then the mixture was ball-milled dry with tungsten carbide balls for 12 hours.

This powder mixture was then annealed at a temperature of 800 C. for 4 hours in vacuum and then compacted at a pressure of tons/sq. in. and sintered in vacuo at 1400 C. for a period of 3 hours. The resulting sintered compacts were then repressed to bring the density up to 85 of the theoretical density and resintered at 1400 C. for 3 hours. Finally, both compacts were consolidated by hot forging, one being then reduced to sheet to form Specimen F and the other being cold-worked to wire and recrystallisation annealed, in accordance with the invention, to form Specimen F.

Creep tests carried out on each of the above, with an applied tensile stress of 700 psi. at a temperature of 1400 C., gave the results summarised below:

lisation Texture. Significant reorientation of grains into the 110 and 2l0 directions had occurred.

F 400 hours.

F 2000 hours. Strong 110 210 Recrystallisation Texture. Significant reorientation of grains into the 110 and 2l0 directions had occurred.

As will be readily appreciated from the above, strengthened platinum, when treated in accordance with this invention, exhibits greatly improved mechanical properties compared With platinum which has not been so treated and has been subjected to the same conditions.

FIGS. 1, 2 and 3 of the accompanying drawings illustrate the recrystallised microstructures of wire from a cast platinum ingot (FIG. 1), sheet made from platinum powder with addition of 0.04% of TiC showing the beginning of an orientated structure (FIG. 2) and finally the highly oriented structure of a wire produced from flake platinum powder containing TiC and recrystallised at 1400 C. after heavy cold-working in accordance with the teaching of this invention (FIG. 3).

Although the invention has been particularly described above in relation to the treatment of strengthened platinum and platinum alloys, it is to be understood that the invention is equally applicable to the treatment of other metals and alloys of the platinum group and also to other noble metals and also to base metals and alloys.

Thus, in addition to the above tests on platinum, further comparative tests were made using copper powder in place of platinum. The powder used for these tests was produced electrolytically of a fineness of less than 200 mesh.

Specimens G and G These specimens were prepared in the following manner: 50 grams of copper powder were reduced in hydrogen at 350-400 C. for a period oi 4 hours. The reduced powder was mixed with 0.04% by weight of micron grade titanium carbide powder and the resulting mixture was then dry milled in a polypropylene mill using steel balls for a period of 12 hours. The powder was then divided into two specimens each of which was annealed in vacuo at 450 C. for 2 hours, made into a compact at a pressure of 10 tons/sq. in., and the compact sintered in vacuo for a period of 4 hours at a temperature of 900950 C. The compacts were then consolidated by gentle cold forging and annealed in hydrogen at a temperature of 500 C. One specimen G was then further reduced by gentle cold-working with frequent intermediate annealing steps, so as to introduce the minimum possible working texture. The other specimen G was subjected to heavy cold-working to effect a reduction of about and then recrystallisation-annealed in accordance with the invention at a temperature of 350 C.

Creep tests carried out on each of the above specimens produced the results summarised below. The tests were each carried out in air at a temperature of 350 C. and under a tensile stress of 5 tons/sq. in.

Specimen: Time to fracture hrs. G 163 G 500 Again, the beneficial results obtainable by means of the invention are clearly obvious and require no further emphasis.

It is also to be understood that the invention is intended to include within its scope any metal or alloy which has been treated by the method of the invention and any article formed of, or incorporating, such metal or alloy.

It is further to be understood that the term coldworking is not intended to be limited to Working at room temperature, as the actual temperature employed will depend to a large extent upon the metal or alloy undergoing the treatment, it being important, however, to ensure that the temperature employed is below that at which significant recrystallisation occurs.

All the possible uses to which the invention may be put are too numerous to recite herein, since the fundamental discovery on which the invention is based, namely, the abiilty greatly to improve the mechanical properties of dispersion strengthened or grain-stabilised metals or alloys by cold-working and subsequent recrystallisation is of universal application in the production of metallic materials for any purpose in which the improved properties attainable by means of the invention are desirable or advantageous. For example, dispersion-strengthened platinum group metals or alloys treated in accordance with the invention may be advantageously employed for the manufacture of apparatus used in the glass industry, such as bushings, used in the production of glass fibres, melting crucibles and stirrers. Another useful application is in the manufacture of resistance thermometers, thermocouples, in which resistance to mechanical failure at high temperatures is important, and electrical resistance furnace windings.

Time to fracture A particularly useful application of the invention, however, will be found to be in the field of catalysts, especially catalyst gauzes, employed in chemical processes, such as the manufacture of nitric acid by oxidation of ammonia or the synthesis of hydrogen cyanide from methane and ammonia.

Catalyst gauzes are usually woven from wire of platinum or a platinum-rhodium alloy, preferably containing or rhodium in the alloy. Wire made of a platinumrhodium alloy is generally preferred owing to its greater mechanical strength compared with that of pure platinum, and, in some instances, its higher catalytic activity compared with the pure metal. For example, in the conversion of methane and ammonia to hydrocyanic acid, by the Andrussov process in which the operating temperatures are in the region of 9001200 C., a catalyst gauze of platinum-rhodium alloy exhibits the same conversion efliciency as pure platinum gauze but the former has the better mechanical properties at the temperatures involved. Conversion efiiciencies of the order of 95-97% can be obtained in the ammonia oxidation process and higher efiiciencies can be obtained by increasing the operating temperature of the catalyst gauze at the expense of the operating life of the catalyst, the maximum temperature at which the gauze can be operated being determined so as to give the catalyst an economically acceptable operating life.

The applicants have found that if the catalyst gauzes are made from platinum or platinum alloy wire composed of strengthened or grain-stabilised platinum or platinum alloy composition which has been treated by the method of this invention, greatly improved mechanical properties are obtained.

The invention is intended, therefore, to include within its scope a catalyst gauze for use in carrying out chemical reactions, which gauze is composed of, or formed from, wire of strengthened platinum or platinum alloy, which has been treated in accordance with this invention and also any catalytic reaction process which involves the use of such a catalyst gauze.

Catalyst gauzes, in accordance with this invention, will be found to oifer considerable advantages over existing catalyst gauzes when used in similar processes and under similar operating conditions. The conversion efficiency of the catalyst will be found to be at least equal to, if not better than, a normal catalyst of a platinum or platinumrhodium alloy wire gauze, whilst the dimensional changes, surface changes and weight changes will be found to be less than those of existing types of catalyst gauzes. Furthermore, a catalyst gauze embodying this invention, will be found to be considerably less subject, at any given temperature, to distortion and creep than are existing platinum or platinum-rhodium catalyst gauzes; the gauze can, therefore, be operated at higher temperatures and will provide higher conversion efiiciencies than is the case 8 with catalyst gauzes which have not been made in accordance with this feature of the invention.

In order to demonstrate the improved results obtained by the use of catalyst gauzes made in accordance with this feature of the invention, the following tests were carried out on gauzes made from:

(a) 10% rhodium-platinum alloy wire treated in accordance with the invention;

(b) 10% rhodium-platinum wire which had not been treated in accordance with the invention, and

(c) Platinum metal wire which had been treated by the method of the invention.

The tests comprised experiments to show:

(1) The effect on the conversion efficiency of the catalyst gauzes of the space velocity of the ammoniaair stream in the manufacture of nitric acid by the oxidation of ammonia on the catalyst gauze;

(2) The eifect of temperature variations on the conversion efficiency of the catalyst gauzes, and

(3) The breaking strength of the catalyst gauzes before and after exposure to the reacting gases.

The results of these tests are given in the following Tables I, II and III and clearly show the improved results obtainable by catalyst gauzes prepared in accordance with the invention.

TABLE I.INFLUENCE OF SPACE VELOCITY ON CONVERSION EFFICIENCY (a) Reoriented grain-stabilised 10% rhodium-platinum: 10% ammonia: gauze temperature 820 C.

Space velocity ft. seer- Conversion efliciency, percent (b) Conventional 10% rhodium-platinum: 10% ammonia: gauze temperature 820 C.

Space velocity ft. secr z Conversion efliciency, percent (c) Reoriented grain stabilised platinum: 10% ammonia: gauze temperature 820 C.

Space velocity ft. secr z Conversion efliciency, percent TABLE II.INFLUENCE OF TEMPERATURE ON CONVERSION EFFICIENCY (a) Reoriented grain-stabilished 10% rhodium-platinum (invention: 10.8% ammonia: 0.348 ft. secr space velocity at N.T.P.

Temperature, C.: Conversion efficiency, percent (b) Conventional 10% rhodium-platinum (prior art): 10.8%ammonia: 0.348 ft. sec? space velocity Temperature, C.: Conversion efliciency, percent 9 (c) Reoriented grain-stabilised platinum (invention): 10.8%ammonia: 0.348 ft. sec.- space velocity Temperature, C.: Conversion efiiciency, percent 820 78.5

TABLE III.BREAKING STRAIN OF CATALYST GAUZES BEFORE AND AFTER REACTION The results given in Table I were obtained using a 10% ammonia-air stream and a catalyst gauze temperature of 820 C.

The results given in Table II were obtained at a gauze temperature of between 660 C. and 840 C. with the use of a 10.8% ammonia-air stream with a space velocity of 0.348 ft./sec. As will be seen from Table III, the room temperatur strength of the gauze embodying the invention was considerably greater than the conventional 10% Rh/ Pt alloy gauze and its strength decreased by only 15.7% compared with a decrease of 59.4% in the case of the untreated alloy gauze.

What is claimed is:

1. A method of improving the mechanical properties of a previously strengthened metal composition comprising a noble metal or copper and including, as a dispersed phase, a strengthening additive selected from the group consisting of refractory metal, carbides, borides, nitrides and silicides, said method comprising the steps of cold-working said composition and subsequently annealing the composition, all of the cold-working being carried out prior to the annealing and the extent of cold working being such that the recrystallization effected during annealing results in an elongated grain structure highly oriented in the direction of working.

2. A method as claimed in claim 1 wherein the noble metal is platinum.

3. A method as claimed in claim 1 wherein strengthened copper power is used.

4. A method as claimed in claim 1 wherein the coldworking is carried out to an extent such as to produce a reduction in cross-sectional area of at least 80".

5. A method as claimed in claim 4 wherein annealing is carried out at a temperature of about 1400 C.

6. The method of claim 1 wherein said strengthened metal composition is a noble metal alloy composition.

7. A method according to claim 6 wherein the noble metal alloy is a platinum-rhodium alloy.

8. A method of improving the mechanical properties of a previously strengthened metal composition comprising a noble metal and including, as a dispersed phase, a strengthening additive selected from the group consisting of refractory metal carbides, borides, nitrides and silicides, said method comprising the steps of cold-working said composition and subsequently annealing said composition, the extent of cold-working being such that the recrystallization effected during annealing results in an elongated grain structure highly oriented in the direction of working.

9. A method as claimed in claim 8 wherein platinum, which has been strengthened by the inclusion therein of is dispersed carbide, is used.

10. The method of claim 8 wherein said metal composition comprises platinum and the additive is titanium carbide.

11. A cold-worked and subsequently annealed composition of improved strength at high temperature, said composition consisting essentially of a noble metal, noble metal alloy, copper or copper alloy and, as a dispersed phase, an additive selected from the group consisting of refractory metal carbide, borides, nitrides and silicides, the composition being annealed only after it has been completely cold-worked and being recrystallized so as to have an elongated grain structure which is highly oriented in the direction of cold-working.

12. A metal composition as claimed in claim 11 where in said metal composition consists of strengthened copper or a copper-base alloy.

13. A cold-Worked and subsequently annealed metal composition of improved strength at high temperature, said composition consisting essentially of a noble metal and, a dispersed phase, an additive selected from the group consists of refractory metal carbides, borides, nitrides and silicides, the composition being recrystallized so as to have an elongated grain structure which is highly oriented in the direction of cold-working.

14. A metal composition as claimed in claim 13 wherein the said metal composition consists of strengthened platinum or a strengthened platinum-rhodium alloy.

References Cited UNITED STATES PATENTS 2,628,926 2/1953 Ramage et al. 148-11.5 2,921,875 I/ 1960 Schnitzel et al 14811.5 3,346,427 10/1967 Baldwin et al. 14811.5 3,366,515 1/1968 Fraser et al 14811.5 3,388,010 6/1968 Stuart et a1 l4811.5 3,399,086 8/1968 Das et al 148-11.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner 

